It is current practice to protect bulk rolled paper and paperboard with a wrapper construction comprising two paper layers between which is sandwiched a moisture vapour barrier. Conventional multi-layer wrappers use low density polyethylene or laminating waxes to provide the requisite moisture vapour barrier qualities.
A major drawback with wax laminated wrappers, however, is that they are difficult to reprocess in conventional repulping operations. Repulping is typically carried out by breaking up the paper product into tiny particles and dissociating the wood fibres from all other materials by flowing the fibres through vibrating screen slots. Contaminants that pass through the screen are subsequently separated from the wood fibres in centrifugal cleaners by differences in specific gravity. Waxes tend to adhere to the wood fibre during the screening process resulting in considerable loss of wood fibre. Waxes themselves are not acceptable materials for reprocessing on a paper machine, as is well known by those familiar with the art of paper making, because their low melting temperature causes deposits to form on paper making machinery. Since screening sequences currently used cannot remove 100% of wax introduced into a system, dispersants and/or solvents are often added to the repulping system to prevent deposition. These additives are expensive and also pose certain problems in terms of toxicity and impact on aquatic life when discharged in a waste stream.
While the use of low density polyethylene and other extruded plastic coatings between two paper or paperboard sheets imparts to the construction a good degree of moisture and vapour barrier protection, such constructions are not easily recyclable. In the repulping stage of the recycling process, the extruded coating tends to separate from the wood fibres into large film pieces. Due to its low density, the film pieces float in the repulper, accumulate around the pulper rotor and plug the screening equipment thus reducing the overall fibre stock preparation process efficiency.
Polyvinylidene chloride (PVdC) is another material which is commonly used as a moisture and vapour barrier. When used in a wet laminating process, it is difficult to achieve a continuous film that provides the moisture and vapour protection specified by the wrapping product end users. Moreover, because PVdC is partially soluble in water, it requires high applied weights to achieve an acceptable level of moisture and vapour protection. The high unit cost of PVdC makes the use of high applied weights not economically viable. Another problem with using high applied weights of PVdC is the negative impact on the environment. Although PVdC dissociates easily from the wood fibres, its chlorine content causes problems in terms of impact on aquatic life when discharged from a recycling facility into a waste effluent stream.
Efforts have been made to reduce the amount of PVdC necessary to achieve adequate moisture and vapour barrier qualities. It is known to coat a thin film of PVdC with polyethylene or polypropylene and to apply the film to the paper substrate. However, the use of polyethylene or polypropylene reduces the repulpability of the product for the reasons stated above.